How to Make It
Preheat the oven to 350°. Butter two 9-inch baking pans and line them with parchment paper.
Set a heatproof bowl over (but not in) a saucepan of simmering water. Add the chopped chocolate and melt, stirring occasionally. Alternatively, melt the chocolate in a microwave. Let cool slightly.
In a medium bowl, whisk the flour with the cocoa powder, baking soda and salt. In a large bowl, beat the 1 1/2 sticks of butter with the sugar until light and fluffy, 3 minutes. Beat in the eggs and egg yolk, one at a time, then beat in the vanilla. At low speed, beat in the buttermilk and the flour mixture in 3 alternating batches, ending with the flour mixture. Fold in the melted chocolate until incorporated.
Scrape the batter into the prepared pans. Bake for about 30 minutes, until a cake tester inserted in the center of the cakes comes out clean. Transfer the cakes to a rack and let cool completely. Turn the cakes out of the pans and peel off the parchment paper.
Toast the pecans on a rimmed baking sheet until golden, 8 to 10 minutes. Let cool slightly, then coarsely chop. In a large saucepan, combine the butter, condensed milk and brown sugar and cook over moderately low heat, whisking frequently, until smooth. Add the egg yolks and cook over moderately low heat, whisking frequently, until thickened, 8 to 10 minutes. Transfer the frosting to a large bowl and stir in 3 cups of the pecans and the coconut, vanilla and salt; let cool.
Using a spoon or offset spatula, spread half of the frosting evenly over one cake. Top with the second cake layer and spread the remaining frosting on top. Garnish with the remaining pecans.